1. Field of the Invention
The present invention pertains to novel organosilicon compositions, e.g., polymers--both crosslinked organosilicon polymers and crosslinkable prepolymers--and to novel nonlinearly optically active dyes. The present invention further relates to novel nonlinearly optically active media, and to novel optical articles, comprising these polymers and dyes, as well as to novel processes for preparing these polymers, dyes, media, and optical articles.
2. Background Information
The significant polarization components of a medium produced by contact with an electro-magnetic field are first order polarization (linear polarization), second order polarization, and third order polarization. On a molecular level, the polarization response (p) of a molecule to the incident electric field (E) can be expressed by the following equation: EQU p=.alpha..multidot.E+.beta.:EE+.gamma.EEE+
wherein .alpha. is the polarizability, .beta. and .gamma. are first and second hyperpolarizabilities, p is the total induced polarization, and E is the local electric field created by an applied electric field. The coefficients in this equation--i.e., .alpha., .beta., etc.--are tensor quantities intrinsic to the molecule under consideration.
For a macroscopic ensemble of molecules, corresponding relationships may be expressed by the following equation: EQU P=.epsilon..sub.0 X.sup.(1) .multidot.E+.epsilon..sub.0 X.sup.(2) :EE+.epsilon..sub.0 X.sup.(3) EEE+
wherein X.sup.(1), X.sup.(2), and X.sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium, .epsilon..sub.0 is the permittivity in vacuum, P is the total induced polarization, and E is the local field created by an applied electric field.
The coefficients X.sup.(1), X.sup.(2), and X.sup.(3) are analogous to .alpha., .beta., and .gamma., except for the fact that they describe a macroscopic assembly of molecules. On both the molecular and the macroscopic levels, the nonlinear optical properties arise from the coefficients for nonlinear polarization--i.e., .beta. and .gamma. at the molecular level, and X.sup.(2) and X.sup.(3) at the macroscopic level; a significant aspect of this invention pertains to those properties emanating from .beta.:EE or X.sup.(2) :EE, which properties are the second order polarization effects.
For second order polarization of a molecule to be of significant magnitude, it is necessary that the first hyperpolarizability (.beta.) be large. A molecule must be non-centrosymmetric for it to possess a nonzero value for .beta.. A large .beta. value is achieved when the molecule exhibits a large difference between ground state and excited state dipole moments, and a large oscillator strength.
In addition to the foregoing requirement on the molecular level, there is an additional symmetry constraint for macroscopic assemblies; a non-zero X.sup.(2) requires a non-centrosymmetric arrangement of the ensemble of molecules. If the symmetry of the charge transfer molecules is broken--i.e., the molecules are arranged so that the dipoles are unidirectionally aligned along one axis--and if this induced order is frozen into place by some means--for example, a polymer matrix, a crystal lattice, etc.--then the macroscopic assembly is capable-of interacting with electro-magnetic waves in such a manner so as to alter the optical properties of the wave--e.g., frequency doubling--and/or the physical properties of the macroscopic assembly--e.g., refractive index changes. These second order nonlinear optical phenomena can be exploited in optoelectronic devices, such as optical switches, phase modulators, amplifiers, and frequency doublers.
Efforts have recently been focused on obtaining efficient second-order nonlinear optical (NLO) properties from poled amorphous polymers. The use of organic polymeric materials offers several advantages over inorganic and crystalline materials. For instance, it is easier to manufacture thin films (e.g., having micrometer thicknesses) of polymer materials onto a variety of substrates for integrated optics applications. An important use of such polymeric thin films incorporates active optical interconnection systems with existing semiconductor electronic technology. This application requires that thin films of the active optical material be deposited onto semiconductor substrates with active semiconductor chips. Crystalline materials, if used in this application, would need controlled methods of growing crystals in the proper molecular orientation and thickness. As a result, the methods of generating these crystalline thin films (e.g., crystallization, molecular organic chemical vapor deposition, liquid phase epitaxy, molecular beam epitaxy) would be time consuming. In contrast, the polymeric thin films are relatively easy to make, using techniques compatible with existing semiconductor processing (i.e., photolithography).
Also to be considered is the nature of the particular component supplying the nonlinear optical response. High nonlinearities, which can exceed those of state of the art inorganics such as lithium niobate, have already been realized in organic dye systems. Because the predominate source of nonlinear optical (i.e., NLO) response of an organic molecule is the polarization of easily perturbed Pi electrons, organic based devices should be able to operate at higher operating frequencies than their inorganic based analogues.
There are four general categories of nonlinear optical media which comprise polymers and organic dyes, both as previously discussed. These are guest/host mixtures, guest/crosslinked host mixtures, polymer bound dyes, and crosslinked polymer bound dyes.
As to these four categories, those systems wherein the nonlinearly optically active organic dye is doped into a linear or crosslinked polymer matrix--i.e., guest/host systems--can achieve high nonlinearities. However, they may be less desirable because there is a time and temperature dependent relaxation of the induced alignment achieved during the poling process; this factor may result in lowered nonlinear optical signals. In addition, high loading of dye in the polymer matrix may be difficult to achieve because of insolubility and phase separation.
Regarding the distinction between these four categories, in guest/host mixtures, the polymer acts simply as a matrix for dissolved dye molecules. In guest/crosslinked host mixtures, the polymer is crosslinked in the presence of a guest dye molecule.
In contrast, a method of generating nonlinearly optically active materials, wherein the dyes may be more permanently aligned, is to covalently bond such dyes into a polymeric matrix.
The polymer bound dyes comprise nonlinear optically active organic dyes covalently bonded to the polymeric chains, either as side groups, or within the backbone itself. In particular, such systems using organosilicon polymers are known in the art.
For instance, FINKELMANN et al., U.S. Pat. No. 4,358,391, CHOE '281, U.S. Pat. No. 4,719,281, LESLIE '659, U.S. Pat. No. 4,801,659, and LESLIE '889, U.S. Pat. No. 4,887,889, all disclose linear polysiloxanes with pendant organic dyes. CHOE '281 additionally discloses the formation of a crosslinked ladder polymer by hydrolysis of a bis(diethoxymethylsilylpropylamine) containing a pendant organic dye.
European Patent Application No. 431,466 discloses cyclic polysiloxanes with pendant NLO organic dyes, and KREUZER et al., U.S. Pat. No. 4,410,570, discloses cyclic polysiloxanes containing mesogenic moieties which exhibit liquid crystalline properties. FUJIMOTO, U.S. Pat. No. 4,698,386, discloses compositions comprising linear and branched polyorganosiloxanes, and dissolved anthraquinone and azo dyes which act as color indicators of curing state. European Patent Application 406,888 discloses a thermosetting silicone resin (polydiphenylsiloxane) containing a guest (not covalently bonded) NLO dye.
Also known in the art are organosilicon polymers which are the hydrosilation product of siloxanes and bicyclic diolefins; further, these resins have been shown to exhibit excellent thermal and electrical properties. For instance, LEIBFRIED '779, U.S. Pat. No. 4,900,779, discloses polymers prepared from reacting polycyclic polyenes, having at least two non-aromatic carbon-carbon double bonds in their rings, with cyclic polysiloxanes and tetrahedral siloxysilanes, having at least two .tbd.SiH groups. The thermoset polymers, disclosed in LEIBFRIED '779, can have glass transition temperatures of about 200.degree. C., and less than 10% weight loss at 500.degree. C. during thermogravimetric analysis; these polymers are also insensitive to water, and resistant to both oxidation and ultraviolet radiation.
It has been discovered that cyclic polysiloxanes with at least two .tbd.SiH groups can undergo hydrosilation with organic dyes having at least two carbon-carbon double bonds--or, optionally, with organic dyes having at least one carbon--carbon double bond and polyenes having at least two non-aromatic carbon--carbon double bonds. It has further been discovered that organosilicon crosslinkable prepolymers and crosslinked polymers can be obtained from this reaction, and that the latter, where the dye components thereof have been sufficiently aligned, exhibit highly stable nonlinear optical properties.
It has also been discovered that, as to organic dyes comprising an electron donor group, an electron acceptor group, and a delocalized Pi electron system linking these groups, such dyes can be obtained having at least two carbon--carbon double bonds pending from two different of these three sites.